Process for preparing substituted 1-o-acyl-2-deoxy-2-fluoro-4-thio-beta-d-arabinofuranoses

ABSTRACT

The present invention relates to a process for preparing 1-O-acyl-2-deoxy-2-fluoro-4-thio-β-D-arabinofuranoses having formula I and intermediates thereof: 
     
       
         
         
             
             
         
       
     
     wherein R 1  represents —C(O)—C 1 -C 6 -alkyl or —C(O)-aryl; and R 2  represents C 1 -C 6 -alkyl, C 1 -C 4 -perfluoroalkyl or aryl.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority to European PatentApplication No. 09075563.8 filed on Dec. 18, 2009 and European PatentApplication No. 10163406.1 filed on May 20, 2010. The entire disclosuresof the applications identified in this paragraph are incorporated hereinby reference.

FIELD

The present invention relates to a process for preparing1-O-acyl-2-deoxy-2-fluoro-4-thio-β-D-arabinofuranoses and intermediatesthereof.

BACKGROUND

4′-Thionucleosides are attractive compounds with respect to antiviraland antineoplastic activity. For example,1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine (4′-thio-FAC)was shown to have excellent antitumour activity in vitro and in vivo [Y.Yoshimura et al; J. Org. Chem. 1997, 62, 3140-3152; S. Miura et al,Cancer Lett. 1998, 129, 103-110; S. Miura et al, Cancer Lett. 1999, 144,177-182; Y. Yoshimura et al, Bioorg. Med. Chem. 2000, 8, 1545-1558; D.A. Zajchowski et al, Int. J. Cancer 2005, 114, 1002-1009].

The invention relates in particular to a novel process for preparingcompounds of formula I

in which R₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl; and R₂represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl.

The compounds of the general formula I are key intermediates in thepreparation of 4′-thionucleosides.

1-(2-Deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine [4′-thio-FAC]:

There is a particular interest in the preparation of1-(2-deoxy-2-fluoro-4-thio-β-D-arabinofuranosyl)cytosine (4″-thio-FAC)with a view to the compound II:

where in the text below the α-diastereomer of the anomeric acetate isreferred to as IIα and IIβ is used for the β-isomer.

This compound and its preparation was described for the first time in WO97/73993, WO 97/038001 and in the literature associated therewith [Y.Yoshimura et al, J. Org. Chem. 1999, 64, 7912-7920; Y. Yoshimura et al,Nucleosides Nucleotides 1999, 18, 815-820; Y. Yoshimura et al, NucleicAcids Symposium Series 1998, 39, 11-12; Y. Yoshimura et al, TetrahedronLett. 1999, 40, 1937-1940].

In this context, a preparation route to compounds of the formula III isdescribed

in which R₃ and R₄ represent alkyl, silyl or acyl, and R₅ representsacyl (Scheme 1).

The starting material in Scheme 1 is the commercially available1,2:5,6-di-O-isopropylidene-α-D-allofuranose (A1) which for its part canbe obtained in four steps from D-glucose [D. C. Baker et al, Carbohydr.Res. 1972, 24, 192-197]. Thus, compound II can be obtained in a total of14 chemical steps starting with1,2:5,6-di-β-isopropylidene-α-D-allofuranose or 18 chemical stepsstarting with D-glucose. Here, compound II is obtained as an anomericmixture consisting of IIα and IIβ. The literature does not make anystatements about the ratio IIα/IIβ [Y. Yoshimura et al, J. Org. Chem.1999, 64, 7912-7920]. Laboratory experiments carried out by theapplicant for preparing II analogously to the literature procedurestarting with A14 gave IIα/IIβ mixtures of 1:1 to 3:2.

A disadvantage of the process, known from the prior art, for preparingthe compound II is the large number of chemical steps required, whichmakes the practice of the process on an industrial scale considerablymore difficult. Furthermore, in particular when the synthesis is carriedout on an industrial scale, there are the following difficulties andproblems:

-   -   The process comprises at least five preparative chromatographic        separations (prep-HPLC).    -   The intermediates A6, A7, A9, A12 are unstable.    -   Handling of the viscous liquids in stages A2, A3, A4, A8, A9,        A11, A12 is difficult.    -   The compound A6 dissolves only very slowly in methanol. In the        presence of sodium methoxide (NaOMe), a nucleophilic        substitution of the mesylate group by a methoxy group in the        compound A7 takes place as a side-reaction. Formation of        by-product takes place in particular when the reaction is        carried out on a relatively large scale.    -   After cleavage of the isopropylidene group, trifluoroacetic acid        (TFA) has to be distilled off under reduced pressure since other        alternatives for work-up result in a large formation of        side-product. On an industrial scale, this is associated with        considerable difficulties.

Owing to the long synthesis sequence and the fact that some of its stepscannot be scaled up, or only with considerable expense, the processshown in Scheme 1 is not suitable for the industrial commercialpreparation of the compound II.

An alternative for preparing the compound II is described in WO2007/068113 and the literature associated therewith [J. K. Watts et al,J. Org. Chem. 2006, 71, 921-925] and is summarized here in Scheme 2.

Here, in the last synthesis step, a mixture of the anomeric acetates IIin a ratio IIα/IIβ of from 1:2 to 1:14 is obtained [see also J. K. Wattset al, J. Org. Chem. 2006, 71, 921-925]. The by-product B8 is removed bycolumn chromatography.

Compound B1 can be prepared in 6 steps from L-xylose (which does notoccur in nature) [J. K. Watts et al, J. Org. Chem. 2006, 71, 921-925].Thus, compound II can be prepared in a total of 13 chemical steps fromL-lyxose.

A particular disadvantage of this synthesis alternative is due to thefact that the starting material L-lyxose is expensive and very little iscommercially available for a synthesis on an industrial scale.

Furthermore, in particular when the synthesis is carried out on anindustrial scale, there are the following difficulties and problems:

-   -   On each synthesis stage, complicated protective group        transformations and in each case a chromatographic purification        have to be carried out.    -   The use of liquid ammonia and elemental lithium at very low        temperatures (step B2).    -   Introduction and removal of a particular silyl protective group        which has a high molar mass and is difficult to obtain        commercially (steps B3 and B5).    -   Use of DAST as fluorinating agent. In addition to the fact that        DAST is difficult to obtain, safety concerns (handling        temperature, decomposition of DAST in an exothermal reaction        with formation of gas) play an important role in the scale-up of        this reaction (step 4).    -   The use of ozone at very low temperatures (B7).    -   High temperatures (110° C.) during the Pummerer rearrangement        and the formation of about 20% of by-product B8 [J. K. Watts et        al, J. Org. Chem. 2006, 71, 921-925].

Owing to the difficulties, described here, in the individual steps ofthe synthesis, which render scale-up difficult or impossible, and owingto the limited availability of the starting material, the process shownin Scheme 2 is likewise not very suitable for the industrial commercialpreparation of the compound II.

SUMMARY

Against this background, it was the object of the present invention toprovide an alternative process allowing the industrial preparation ofcompounds of formula I.

According to the invention, this object was achieved by a process whichaffords compounds of formula I in high yields in 10 chemical stepsstarting with commercially readily available compounds of formula IV viathe key steps “introduction of the fluorine atom via targeted opening ofa cyclic sulphate” and “Pummerer rearrangement of a sulphoxide using aspecial catalyst” (Scheme 3).

Compounds of formula IV, used as starting materials for the processaccording to the invention, can be synthesized in 4 chemical steps fromnatural and thus readily available D-ribose. Both D-ribose and compoundsof the type IV (for example5-O-benzyl-2,3-O-isopropylidene-L-lyxono-1,4-lactone) are commerciallyavailable (U.S. Pat. No. 6,448,415 B1).

Furthermore, the process according to the invention uses exclusivelyreagents which are readily available even in kg amounts.

The intermediates V, VI and VII are isolated only as crude products andin each case employed directly for the next step. Finally, the compoundVIII is crystallized in high purity (>97%). Thus, time- andresource-intensive purifications (for example preparativechromatography) can be dispensed with.

Steps IX, XI and XII are likewise not isolated and directly used ascrude materials for the subsequent step.

According to the invention, over the entire synthesis sequence, only 3intermediates (VIII, X and XIII) have to be isolated, and only a singlepreparative chromatography is required (X). The intermediates VIII, XIIand the product I (II) are isolated in high yields and high purities(>93%) by crystallization.

The process according to the invention does not require any complicatedprotective group transformations.

The oxidation of the sulphide XII can be carried out in a targetedmanner at room temperature using OXONE (potassium monopersulphate triplesalt, 2 KHSO₅*KHSO₄*K₂SO₄), overoxidation can be excluded without anyproblems by using equimolar amounts (cf. Scheme 2, reaction yieldingB7).

According to the invention, the Pummerer rearrangement (XIII→I) iscarried out in the presence of catalytic amounts of potassiumbisulphate. By using this catalyst, it is possible to achieve highyields (>80%) and at the same time very little side-product formation(<5%) at low reaction temperatures (<90° C.) [cf. Scheme 2, reactionyielding II]. The crude product I obtained comprises so few impuritiesand also only very small proportions of the α-anomer that a simplecrystallization is sufficient for purification.

The process according to the invention does make use of general chemicaltransformations, known to the person skilled in the art, forconstructing a thiofuranose via formation of a diol, its activation viaa bissulphonate and cyclization with sodium sulphide; for protectivegroup techniques and for oxidizing a sulphide with OXONE. However, aparticular aspect of the present invention is the targeted and highlyefficient construction of the individual stereocentres of thethiofuranoses.

A further aspect of the present invention is the stereospecificintroduction of the fluorine atom at the C3 atom of compound X viastereoselective opening of the cyclic sulphate IX.

DETAILED DESCRIPTION

In the first step of the process according to the invention forpreparing a compound of formula I:

in whichR₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl; andR₂ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl,a lyxonolactone of formula IV:

in whichR₆ represents C₁-C₆-alkyl or arylmethylene; andR₇ and R₈ independently of one another represent hydrogen, C₁-C₆-alkyl,C₁-C₄-perfluoroalkyl or aryl;is reduced in the presence of 0.5-10 molar equivalents of hydride donorsof the formula A(AlH₄) or A(BH₄), in which A represents an alkali metal,to give the diol of formula V:

Here, preference is given to using 0.5-1.5 molar equivalents of lithiumaluminium hydride (LiAlH₄). The reaction step is preferably carried outat a temperature between 0° C. and 30° C.

In the second step, the diol V is reacted with at least 2 molarequivalents of sulphonyl chloride R₉—SO₂Cl or sulphonic anhydrideR₉—SO₂—O—SO₂—R₉, in which R₉ represents C₁-C₆-alkyl,C₁-C₄-perfluoroalkyl or aryl, preferably 2-5 molar equivalents ofmethanesulphonyl chloride, in the presence of at least 2 molarequivalents of a tertiary amine (for example trimethylamine,triethylamine, diisopropylethylamine) or a pyridine (for examplepyridine, 4-N,N-dimethylaminopyridine, collidine, picolines, lutidines),preferably 2-5 molar equivalents of triethylamine, to give a compound offormula VI:

The reaction step is preferably carried out at a temperature between 0°C. and 30° C.

In the third step, the bissulphonate VI is reacted with at least 1 molarequivalent of sodium sulphide (Na₂S) in a polar aprotic solvent, suchas, for example, DMF, NMP, DMA, DMSO, DMEU, preferably NMP(N-methylpyrrolidone), at a temperature of more than 50° C., preferablyat temperatures between 50 and 100° C., to give the thiofuranose offormula VII:

In step four, the thiofuranose of formula VII is converted in a solventmixture of water and an organic solvent selected from the group of theethers (for example diethyl ether, THF, dioxane, MTBE), alcohols (forexample methanol, ethanol, isopropanol), carboxylic acids (for exampleacetic acid), aromatic hydrocarbons (for example benzene, toluene),preferably in a mixture of tetrahydrofuran (THF) and water, with 0.01-5molar equivalents of an acid selected from the group of the mineralacids (for example HCl, H₂SO₄, H₃PO₄), alkanesulphonic acids (forexample methanesulphonic acid), arylsulphonic acids (for examplebenzenesulphonic acid, toluenesulphonic acid), perfluorosulphonic acids(for example trifluoromethanesulphonic acid, nonafluorobutanesulphonicacid) or perfluoroalkanecarboxylic acids (for example trifluoroaceticacid), preferably 0.01-5 molar equivalents of H₂SO₄, into the diol offormula VIII:

The reaction step is preferably carried out at a temperature between 20°C. and 100° C.

The diol VIII is isolated by crystallization from a solvent mixture ofan alkane or mixtures thereof—preferably heptane—and a carboxylicester—preferably isopropyl acetate or ethyl acetate.

In step five, the diol of general formula VIII is reacted in thepresence of at least 0.2 molar equivalents of a base of the formula AH,A₂CO₃ or A(OtBu), in which A represents an alkali metal; preferably inthe presence of 0.2-3 molar equivalents of sodium hydride, with 1-2molar equivalents of a diol-activating reagent of the formula X₁—SO₂—X₂,in which X₁ and X₂ independently of one another represent Cl orimidazoyl, preferably with 1-2 molar equivalents ofsulphonyldiimidazole, to give the cyclic sulphate ester of formula IX:

The reaction step is preferably carried out at a temperature between—5°C. and 20° C.

In step six, the cyclic sulphate ester of formula IX is initiallyreacted with 1-3 molar equivalents of an ammonium fluoride of theformula N(R₁₁)₄F, in which R₁₁ represents C₁-C₆-alkyl, preferably with1-3 molar equivalents of tetrabutylammonium fluoride, at a temperaturebetween 0° C. and 30° C., and the reaction mixture obtained in thismanner is reacted with an acid selected from the group of the mineralacids (for example HCl, H₂SO₄), alkylsulphonic acids (for examplemethanesulphonic acid), arylsulphonic acids (for examplebenzenesulphonic acid, toluenesulphonic acid), perfluorosulphonic acids(for example trifluoromethanesulphonic acid, nonafluorobutanesulphonicacid) or perfluoroalkanecarboxylic acids (for example trifluoroaceticacid), preferably with sulphuric acid (H₂SO₄), to give the ether offormula X:

The second partial step of the reaction is preferably carried out at atemperature between 20° C. and 70° C.

In step seven, the ether of the formula X is initially, in partial stepa), reacted in the presence of at least 1 molar equivalent of a boronhalide BY₃, in which Y represents F, Cl or Br, preferably using 1-4molar equivalents of boron trichloride, at a temperature between 0° C.and −80° C.

In partial step b), the reaction mixture obtained in partial step a) isreacted with a mixture of

an alcohol component selected from the group consisting of aC₁-C₆-alkanol (for example methanol, ethanol, n-propanol, isopropanol,n-butanol, t-butanol, isobutanol), an arylalkanol (for example benzylalcohol) and phenols (for example phenol); and a base selected from thegroup of aliphatic tertiary amines (for example trimethylamine,triethylamine, diisopropylethylamine) or from the group of pyridines(for example pyridine, 4-N,N-dimethylaminopyridine, collidine,picolines, lutidines); preferably using a mixture of methanol andpyridine, to give the diol of formula XI:

The process and quenching of the reaction mixture are preferably carriedout at temperatures between 0° C. and −80° C.

In step eight, the diol of formula XI is reacted in the presence of atleast 2 molar equivalents of a base selected from the group of thealiphatic tertiary amines (for example trimethylamine, triethylamine,diisopropylethylamine) or from the group of the pyridines (for examplepyridine, 4-N,N-dimethylaminopyridine, collidine, picolines, lutidines),preferably using 2-10 molar equivalents of pyridine, with at least 2molar equivalents of an acid chloride R₁—Cl or an acid anhydrideR₁—O—R₁, in which R₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl;preferably using 2-5 molar equivalents of benzoyl chloride, to give thecompound of formula XII:

In step nine, the sulphide of formula XII is oxidized in a solventmixture of water and a ketone of the formula R₉—C(O)—R₉′ in which R₉ andR₉′ independently of one another represent C₁-C₆-alkyl,C₁-C₄-perfluoroalkyl or aryl, preferably acetone, with 0.5-1 molarequivalent of an alkali metal persulphate of the formula AHSO₅, in whichA⁺ represents an alkali metal, preferably using 0.5-1 molar equivalentof OXONE (potassium monopersulphate triple salt, 2 KHSO₅*KHSO₄*K₂SO₄),at a temperature between 0° C. and 50° C. to give the sulphoxide offormula XIII:

The product XIII is isolated by crystallization from a suitable solvent,preferably from methyl tert-butyl ether.

In step ten, the sulphoxide of formula XIII is reacted with at least 1molar equivalent of an acid anhydride R₂—C(O)—O—C(O)₂, in which R₂represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl; preferably usingat least 5 molar equivalents of acetic anhydride, in the presence of0.01-2 molar equivalents of a protic acid selected from the group of themineral acids (for example HCl, HBr, H₂SO₄, H₃PO₄, alkali metalbisulphates, monobasic alkali metal phosphates), alkanesulphonic acids(for example methanesulphonic acid), arylsulphonic acids (for examplebenzenesulphonic acid, toluenesulphonic acid), perfluorosulphonic acids(for example trifluoromethanesulphonic acid, nonafluorobutanesulphonicacid) or perfluoroalkanecarboxylic acids (for example trifluoroaceticacid), or in the presence of 0.01-2 molar equivalents of a Lewis acid(for example LiCl, MgBr₂, Ti(OR₁₃)₄), in which R₁₃ representsC₁-C₆-alkyl or arylmethylene; preferably in the presence of 0.01-2 molarequivalents of potassium bisulphate; at a temperature between 30° C. and100° C. to give the compound of formula I:

The product I is isolated by crystallization from a suitable solvent,preferably ethanol.

Hitherto, the literature does not provide any examples in whichpotassium bisulphate catalyses the Pummerer rearrangement of asulphoxide to the corresponding thioacetal and prevents the formation ofside-products.

The C₁-C₆-alkyl groups of the radicals R₁, R₂, R₆, R₇, R₈, R₉, R₉′, R₁₁,and R₁₃ can, for example, be methyl, ethyl, n-propyl, isopropyl,n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl or n-hexyl groups.

The arylmethylene groups of the radicals R₆, R₉, and R₁₃ can, forexample, be benzyl or 4-methoxybenzyl groups.

The aryl groups of the radicals R₁, R₂, R₇, R₈, R₉, and R₉′ can, forexample, be phenyl or substituted phenyl groups.

The C₁-C₄-perfluoroalkyl groups of the radicals R₂, R₇, R₈, R₉, and R₉′can, for example, be trifluoromethyl, pentafluoroethyl ornonafluorobutyl groups.

The alkali metals A of the hydride donors of the formulae A(AlH₄),A(BH₄), of the bases AH, A₂CO₃, AOtBu, of the alkali metal persulphatesAHSO₅ can, for example, be lithium, sodium or potassium.

The —C(O)—C₁-C₆-alkyl groups of the radical R₁ can, for example, beacetyl, n-propanoyl, isopropanoyl, n-butanoyl, t-butanoyl, isobutanoyl,n-pentanoyl, or n-hexanoyl groups.

The —C(O)-aryl groups of the radical R₁ can, for example, be benzoyl orsubstituted benzoyl groups.

Preferably, the process according to the invention is used for preparingthe compound I (according to Scheme 3) in which R₁ represents benzoyl,and R₂ represents methyl, i.e. for preparing the compound of formulaIIβ.

According to the present invention, particular preference is given tothe process for preparing the compound of formula IIβ (according toScheme 3) in which R₇, R₈ and R₉ represent methyl, and R₆ representsbenzyl.

Furthermore, the present invention also relates to the intermediates ofthe preferred embodiment of the process according to the invention(Scheme 4), in particular

-   1-O-benzyl-3,4-O-isopropylidene-L-arabinitol (C2);-   1-O-benzyl-3,4-O-isopropylidene-2,5-di-O-methanesulphonyl-L-arabinitol    (C3);-   1,4-anhydro-5-O-benzyl-2,3-O-isopropylidene-4-thio-D-ribitol (C4);-   1,4-anhydro-5-O-benzyl-2,3-O-sulphonyl-4-thio-D-ribitol (C6);-   1,4-anhydro-5-O-benzyl-2-deoxy-2-fluoro-4-thio-D-arabinitol (C7).

The compound C1 (5-O-benzyl-2,3-O-isopropylidene-L-lyxono-1,4-lactone)is commercially available. Hereinbelow, the procedures for the reactionsshown in Scheme 4 are described.

Synthesis Procedures 1-O-Benzyl-3,4-O-isopropylidene-L-arabinitol (C2)

At 0-10° C., 44.68 g (116.78 mmol) of lithium aluminium hydride aremetered into a solution of 50 g (179.66 mmol) of C1 in 450 ml of THF,and the mixture is stirred until the reaction has ended. At 20° C.,water and aqueous sodium hydroxide solution are then added, and theprecipitated solid is filtered off and washed product-free using THF.The solution of the crude product is concentrated and freed completelyfrom the solvent. This gives 53.26 g of C2 (105%) as a crude productwhich is used in this form for the subsequent step C3.

¹H-NMR (400 MHz, DMSO): 7.39-7.24 (m, 5H, H-7 to H-9), 4.81 (t, 1H, 5.46Hz, 1-OH), 4.65 (d, 1H, 5.84 Hz, 4-OH), 4.5 (s, 2H, 2×H-6), 4.15-4.00(m, 2H, H-2 and H-3), 3.76 (qd, 1H, 6 Hz, 2.5 Hz, H-4), 3.7-3.55 (m, 2H,2×H-1), 3.49-3.35 (m, 2H, 2×H-5), 1.38 (s, 3H, 10-CH₃), 1.25 (s, 3H,10-CH3).

¹³C-NMR (75 MHz, CDCl₃): 138.0, 128.8, 128.2, 128.1 (Ar), 108.9 (O—C—O,77.7 (CH2), 76.9 (CH2), 73.8 (CH), 71.9 (CH), 68.9 (CH2), 61.0 (CH),28.1 (CH3), 25.6 (CH3).

1-O-Benzyl-3,4-O-isopropylidene-2,5-di-O-methanesulphonyl-L-arabinitol(C3)

At 20° C., 54.54 g (538.97 mmol) of triethylamine are added to asolution of 53.26 g (179.66 mmol assuming a yield of 100% in step C2) ofC2 (crude product) in 450 ml of THF, and 39.39 g (431.17 mmol) ofmethanesulphonyl chloride are metered in at 0-10° C. The mixture is thenstirred until the reaction has ended, water is added and the phases areseparated. The aqueous phase is extracted with MTBE, and the combinedorganic phases are washed with sat. sodium chloride solution and dilutesodium bicarbonate solution. The solution of the crude product isconcentrated and freed completely from the solvents. This gives 86.66 gof C3 (110%) as a crude product which is used in this form for thesubsequent step C4.

¹H-NMR (400 MHz, CDCl₃): 7.40-7.30 (m, 5H), 4.90 (dt, 1H, 11.5 Hz, 6.6Hz), 4.56 (dd, 2H, 11.8 Hz), 4.42-4.39 (m, 2H), 4.38-4.35 (m, 2H), 3.83(dd, 1H, 10.61 Hz, 6.06 Hz), 3.70 (dd, 1H, 10.61 Hz, 5.05 Hz), 3.10 (s,3H), 3.02 (s, 3H), 1.50 (s, 3H), 1.37 (s, 3H).

¹³C-NMR (75 MHz, CDCl₃): 136.8, 128.6, 128.2, 128.0 (Ar), 109.7 (O—C—O),77.8. 75.6, 74.5, 73.7 (CH2), 69.6 (CH2), 67.7 (CH2), 39.0, 37.5, 27.2,25.4.

1,4-Anhydro-5-O-benzyl-2,3-O-isopropylidene-4-thio-D-ribitol (C4)

50.99 g (215.59 mmol) of sodium sulphide (33% pure) are added to asolution of 86.66 g (179.66 mmol assuming a yield of 100% in step C3) ofC3 (crude product) in 550 ml of N-methyl-2-pyrrolidone, and the mixtureis heated to an internal temperature of 80° C. and stirred at thistemperature until the reaction has ended. At an internal temperature of20° C., water and MTBE are then added, the phases are separated and theaqueous phase is extracted with MTBE. The combined organic phases arefinally washed with water. The solution of the crude product isconcentrated and freed completely from the solvents. This gives 56.41 gof C4 (112%) as a crude product which is used in this form for thesubsequent step C5.

¹H-NMR (400 MHz, DMSO): 7.40-7.25 (m, 5H, H-7 to H-9), 4.89 (t, 1H, 4.7Hz, H-2), 4.73 (d, 1H, 5.65 Hz, H-3), 4.50 (s, 2H, 2×H-6), 3.55-3.42 (m,2H, 2×H-5), 3.37 (t, 1H, 6.2 Hz, H-4), 3.10 (dd, 1H, 12.6 Hz, 4.7 Hz,H-1), 2.76 (d, 1H, 12.6 Hz, H-1′).

¹³C-NMR (75 MHz, CDCl₃): 137.9, 128.4, 127.9, 127.5 (Ar), 110.9 (O—C—O),86.3 (CH), 83.8 (CH), 73.2 (CH2), 72.2 (CH2), 53.3 (CH), 38.3 (CH2),27.2 (CH3), 25.3 (CH3).

1,4-Anhydro-5-O-benzyl-4-thio-D-ribitol (C5)

A solution of 50 ml of water and 9.22 g (0.094 mmol) of sulphuric acidis added to the solution of 56.41 g (179.66 mmol assuming a yield of100% in step C4) of C4 (crude product) in 450 ml of THF, and the mixtureis heated to 70° C. and stirred at this temperature for a plurality ofhours. To achieve complete conversion, a few ml are distilled off at 70°C. MTBE is then added at 20° C., the phases are separated and theaqueous phase is extracted with MTBE. The combined organic phases areneutralized with saturated potassium carbonate solution. The solid isfiltered off and washed product-free using MTBE. The solution of thecrude product is redistilled to isopropyl acetate, and the product iscrystallized by addition of heptane. This gives 33.67 g (78% over 4steps starting with C1) of C5.

¹H-NMR (400 MHz, CDCl₃): 7.39-7.29 (m, 5H), 4.56 (s, 2H), 4.37 (dt, 1H,7.33 Hz, 3.54 Hz), 4.04 (dt, 1H, 7.07 Hz, 6.82 Hz), 3.70 (dd, 1H, 9.09Hz, 5.31 Hz), 3.62 (t, 1H, 9.09 Hz), 3.56-3.51 (m, 1H), 3.11 (d, 1H,3.54 Hz), 3.05 (dd, 1H, 11.62 Hz, 4.55 Hz), 2.85 (dd, 1H, 11.62 Hz, 3.03Hz), 2.65 (d, 1H, 3.54 Hz).

¹³C-NMR (75 MHz, CDCl₃): 137.4, 128.6, 128.0, 127.8 (Ar), 80.4, 74.8,73.6 (CH2), 73.0 (CH2), 47.0, 33.6 (CH2).

Melting point: 78-82° C.

1,4-Anhydro-5-O-benzyl-2,3-O-sulphonyl-4-thio-D-ribitol (C6)

At 0° C., a solution of 50 g (208.06 mmol) of C5 in 75 ml of THF isadded to a suspension of 2.5 g of sodium hydride (in mineral oil) in 100ml of THF, and the mixture is stirred at 0° C. for 1-2 hours. At 0° C.,a solution of 45.36 g of sulphonyldiimidazole in 450 ml of THF is thenmetered in, and the mixture is stirred at 20° C. until the reaction isended. In this form, the crude product is used directly for thesubsequent step C7.

¹H-NMR (400 MHz, CDCl₃): 7.40-7.26 (m, 5H), 5.41 (dt, 1H, 6.06 Hz, 3.28Hz), 5.33 (dd, 1H, 6.32 Hz, 2.78 Hz), 4.54 (d, 2H, 2.53 Hz), 3.81 (dd,1H, 9.85 Hz, 4.29 Hz), 3.72 (m, 1H), 3.64 (dd, 1H, 9.85 Hz, 4.80 Hz),3.47 (dd, 1H, 13.39 Hz, 5.81 Hz), 3.16 (dd, 1H, 13.39 Hz, 3.28 Hz).

¹³C-NMR (75 MHz, CDCl₃): 137.0, 128.6, 128.2, 127.7 (Ar), 87.9, 86.1,73.7 (CH2), 71.2 (CH2), 51.6, 36.8 (CH2).

1,4-Anhydro-5-O-benzyl-2-deoxy-2-fluoro-4-thio-D-arabinitol (C7)

At 20° C., a solution of 131.3 g of tetrabutylammonium fluoridetrihydrate in 150 ml of THF is metered into the solution of the crudeproduct from the preparation of compound C6. The mixture is stirred at30° C. until the reaction is ended.

The reaction mixture is then adjusted to pH 1 using sulphuric acid (33%strength) and stirred at 50° C. until the reaction is ended. At 20° C.,the pH is then adjusted to 7-10 using aqueous potassium hydroxidesolution, and the precipitate formed is filtered off. The phases areseparated and the aqueous phase is extracted with dichloromethane. Thecombined organic phases are concentrated and the crude product is, aftercomplete removal of the solvent, purified by chromatography. This gives22.68 g of C7 (45%).

¹H-NMR (400 MHz, CDCl₃): 7.4-7.27 (m, 5H, H-7 to H-9), 5.04 (qd, 1H,51.7 Hz, 5.44 Hz, H-2), 4.55 (s, 2H, 2×H-6), 4.37 (td, 1H, 11.7 Hz, 4.9Hz, H-3), 3.68-3.55 (m, 2H, 2×H-5), 3.45-3.38 (m, 1H, H-4), 3.2-2.99 (m,2H, 2×H-1).

¹³C-NMR (100 MHz, CDCl₃): 137.65, 128.52, 127.91, 127.77 (Ar), 97.27(C-2), 79.10 (C-3), 73.46 (C-6), 72.69 (C-5), 48.58 (C-4), 31.72 (C-1).

¹⁹F-NMR (376 MHz, CDCl₃): −183.14 (m, 2-F).

Melting point: 83-85° C.

1,4-Anhydro-2-deoxy-2-fluoro-4-thio-D-arabinitol (C8)

At a temperature of <−65° C., a solution, pre-cooled to −10° C., of 50 g(206.35 mmol) of C7 in 350 ml of dichloromethane is added to 546.82 g(412.69 mmol) of boron trichloride (1 mol/l in dichloromethane). After30 minutes at <−65° C., the reaction is checked for complete conversion.A mixture of 150 ml of methanol and 116 ml of pyridine is then added at<−65° C. to the reaction mixture, and the mixture is heated to 20° C.after 15 minutes. The solvents are removed completely by distillationunder reduced pressure. Dichloromethane is then added to the residue,and the solvent is removed completely. This gives 133 g of C8 (423%) asa crude product (contains residual amounts of pyridine and pyridiniumhydrochloride) which is used in this form for the subsequent step C9.

¹H-NMR (400 MHz, DMSO): 5.49 (d, 1H, 4.77 Hz, 3-OH), 4.98 (qd, 1H, 51.55Hz, 3.76 Hz, H-2), 4.94 (t, 1H, 5.2 Hz, 5-OH), 4.17 (ddd, 1H, 15.18 Hz,4.14 Hz, 3.76 Hz H-3), 3.61-3.53 (m, 1H, H-5), 3.37-3.30 (m, 1H, H-5′),3.19-3.02 (m, 2H, H-4 and H-1), 2.93 (ddd, 1H, 18.22 Hz, 12.1 Hz, 3.76Hz, H-1′).

¹³C-NMR (75 MHz, CDCl₃): 97.1 (d, 185 Hz, C-2), 77.5 (d, 24 Hz, C-3),63.3 (d, 3 Hz, C-5), 50.9 (d, 4 Hz, C-4), 31.1 (d, 22 Hz, C-1).

1,4-Anhydro-2-deoxy-2-fluoro-3,5-di-O-benzoyl-4-thio-D-arabinitol (C9)

133 g (206.35 mmol assuming a yield of 100% in step C8) of C8 (crudeproduct) are dissolved in 400 ml of dichloromethane, and 97.92 g (1237.9mmol) of pyridine are added. 87.01 g (618.97 mmol) of benzoyl chlorideare then added dropwise at 10° C., and the mixture is stirred at 20° C.until the reaction has ended. Methanol is then added, and the mixture isstirred for 1 hour. Finally, water is added and the phases areseparated. The solution of the crude product is concentrated and freedcompletely from the solvent. This gives 114.53 g of C9 (154%) as a crudeproduct which is used in this form for the subsequent step C10.

¹H-NMR (400 MHz, CDCl₃): 8.20-8.00 (m, 4H), 7.65-6.90 (m, 6H), 5.85 (dt,1H, 9.98 Hz, 2.64 Hz), 5.40 (ddd, 1H, 48.79 Hz, 7.16 Hz, 3.01 Hz), 4.54(s, 1H), 4.51 (s, 1H), 3.95-3.82 (m, 1H), 3.45-3.35 (m, 1H), 3.35-3.28(m, 1H).

¹³C-NMR (75 MHz, CDCl₃): 165.0 (C═O), 133.6, 133.1, 129.8, 129.7, 128.5,128.4, 96.5 (d, 184 Hz, C-2), 79.0 (d, 29 Hz, C-3), 65.2 (d, 4 Hz, C-5),48.5 (C-4), 34.9 (d, 23 Hz, C-1).

1,4-Anhydro-2-deoxy-2-fluoro-3,5-di-O-benzoyl-4-sulphinyl-D-arabinitol(C10)

114.53 g (206.35 mmol assuming a yield of 100% in step C9) of C9 (crudeproduct) are dissolved in 400 ml of acetone, and 60 ml of water areadded. At 20° C., 69.77 g (113.49 mmol) of OXONE are then added a littleat a time. The reaction is checked for complete conversion, and a dilutesolution of sodium sulphite is then added. The reaction mixture isneutralized using saturated sodium bicarbonate solution, and the acetoneis then removed completely by distillation under reduced pressure.Dichloromethane is added to the suspension, the solid is filtered offand the filtercake is washed product-free with dichloromethane. Thesolution of the crude product is redistilled to MTBE and isolated inthis solvent. This gives 56.4 g (72.6% over three steps with C7) of C10.

¹H-NMR (400 MHz, CDCl₃): 8.05 (m, 4H), 7.60 (m, 2H), 7.45 (m, 4H), 5.83(m, 1H), 5.74 (m, 1H), 4.89 (ddd, 1H, 12.1 Hz, 5.1 Hz, 0.9 Hz), 4.75(ddd, 1H, 12.4 Hz, 7.6 Hz, 0.9 Hz), 3.65 (m, 1H), 3.75 (m, 1H), 3.45 (m,1H).

¹³C-NMR (75 MHz, CDCl₃): 165.7, 165.2 (C═O), 134.0, 133.5, 130.0, 129.7,128.6, 128.5 (Ar), 95.4 (d, 185 Hz, C-2), 77.2 (d, 33 Hz, C-3), 71.6(C-4), 61.1 (d, 2 Hz, C-5), 55.8 (d, 19 Hz, C-1).

1-O-Acetyl-2-deoxy-2-fluoro-3,5-di-O-benzoyl-4-thio-β-D-arabinofuranose(IIβ)

80 ml of acetic anhydride and 361 mg (2.66 mmol) of potassium bisulphateare added to 10 g (26.57 mmol) of C10, and the mixture is stirred at 80°C. until the reaction is ended. The reaction mixture is then codistilledinitially repeatedly with toluene and then with ethanol. Finally, theproduct is crystallized from ethanol. This gives 8.9 g (80%) of thecompound IIβ.

¹H-NMR (400 MHz, CDCl₃): 8.10-7.90 (m, 4H, Ar), 7.63-7.29 (m, 6H, Ar),6.18 (d, 1H, 4.4 Hz), 6.12-6.02 (m, 1H), 5.45 (dd, 1H, 9.04 Hz, 4.52Hz), 5.28 (dd, 1H, 8.85 Hz, 4.52 Hz), 4.68 (dd, 1H, 11.49 Hz, 6.22 Hz),4.49 (dd, 1H, 11.49 Hz, 6.41 Hz), 3.74 (dd, 1H, 13.56 Hz, 6.40 Hz), 2.12(s, 3H).

¹³C-NMR (75 MHz, CDCl₃): 169.6 (COCH₃), 165.8, 165.4 (COPh), 133.6,133.1 (Ar) 129.8, 129.7, 128.5, 128.2 (Ar), 92.5 (d, 207 Hz, C-2), 75.7(d, 23 Hz, C-3), 74.0 (d, 17 Hz, C-1), 66.1 (C-5), 42.4 (d, 7 Hz, C-4),21.0 (CH₃).

Melting point: 130° C.

The process of the present invention provides an industriallyadvatageous and excellent preparation of the compounds of the formula I.

1. A process for preparing a compound of formula I

wherein R₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl; and R₂ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl, which comprises the following steps: (1) reduction of a lyxonolactone of formula IV

wherein R₆ represents C₁-C₆-alkyl or arylmethylene; and R₇ and R₈ independently represent hydrogen, C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl; in the presence of 0.5-10 molar equivalents of hydride donors of the formula A(AlH₄) or A(BH₄), wherein A is an alkali metal, to give a diol of formula V:

(2) reaction of the diol of formula V with at least 2 molar equivalents of sulphonyl chloride R₉—SO₂Cl or sulphonic anhydride R₉—SO₂—O—SO₂—R₉, wherein R₉ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl, in the presence of at least 2 molar equivalents of a tertiary amine or a pyridine, to give a compound of formula VI:

(3) reaction of the bissulphonate of formula VI with at least 1 molar equivalent of sodium sulphide (Na₂S) in a polar aprotic solvent, at a temperature of more than 50° C., to give a thiofuranose of formula VII:

(4) conversion of the thiofuranose of formula VII in a solvent mixture of water and an organic solvent selected from the group of ethers, alcohols, and aromatic hydrocarbons, with 0.01-5 molar equivalents of an acid selected from the group of mineral acids, alkanesulphonic acids, arylsulphonic acids, perfluorosulphonic acids and perfluoroalkanecarboxylic acids, into a diol of formula VIII:

(5) reaction of the diol of formula VIII in the presence of at least 0.2 molar equivalents of a base of the formula AH, A₂CO₃ or A(OtBu), wherein A represents an alkali metal; with 1-2 molar equivalents of a diol-activating reagent of the formula X₁—SO₂—X₂, wherein X₁ and X₂ independently represent Cl or imidazoyl, to give a cyclic sulphate ester of formula IX:

(6) the cyclic sulphate ester of formula IX is initially reacted with 1-3 molar equivalents of an ammonium fluoride of the formula N(R₁₁)₄F, wherein R₁₁ represents C₁-C₆-alkyl, at a temperature between 0° C. and 30° C., and the reaction mixture obtained in this manner is reacted with an acid selected from the group of mineral acids, alkylsulphonic acids, arylsulphonic acids, perfluorosulphonic acids and perfluoroalkanecarboxylic acids to give an ether of formula X:

(7) the ether of formula X is in partial step a) reacted in the presence of at least 1 molar equivalent of a boron halide BY₃, wherein Y represents F, Cl or Br, at a temperature between 0° C. and −80° C.; in partial step b), the reaction mixture obtained in partial step a) is reacted with a mixture of an alcohol component selected from the group consisting of C₁-C₆-alkanol, aryl alkanol and phenols; and a base selected from the group of aliphatic tertiary amines and pyridines; to give a diol of formula XI:

(8) the diol of formula XI is in the presence of at least 2 molar equivalents of a base selected from the group of aliphatic tertiary amines and pyridines, reacted with at least 2 molar equivalents of an acid chloride R₁—Cl or an acid anhydride R₁—O—R₁, wherein R₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl; to give a compound of formula XII:

(9) the sulphide of formula XII is oxidized in a solvent mixture of water and a ketone of the formula R₉—C(O)—R₉′, wherein R₉ and R₉′ independently represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl, with 0.5-1 molar equivalent of an alkali metal persulphate of the formula AHSO₅, wherein A⁺ represents an alkali metal, at a temperature between 0° C. and 50° C. to give a sulphoxide of formula XIII:

(10) the sulphoxide of formula XIII is reacted with at least 1 molar equivalent of an acid anhydride R₂—C(O)—O—C(O)—R₂, wherein R₂ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl; in the presence of 0.01-2 molar equivalents of a protic acid selected from mineral acids, alkanesulphonic acids, arylsulphonic acids, perfluorosulphonic acids and perfluoroalkanecarboxylic acids, or in the presence of 0.01-2 molar equivalents of a Lewis acid selected from LiCl, MgBr₂ and Ti(OR₁₃)₄, wherein R₁₃ represents C₁-C₆-alkyl or arylmethylene; at a temperature between 30° C. and 100° C.
 2. The process according to claim 1 for preparing a compound of formula I wherein R₁ represents benzoyl, and R₂ represents methyl.
 3. The process according to claim 2 for preparing a compound of formula IIβ.

wherein R₇, R₈ and R₉ represent methyl, and R₆ represents benzyl.
 4. The process according to claim 1, wherein in step (1) 0.5-1.5 molar equivalents of lithium aluminium hydride (LiAlH₄) are used.
 5. The process according to claim 1, wherein the step (1) and/or the step (2) are carried out at a temperature between 0° C. and 30° C.
 6. The process according to claim 1, wherein in step (2) 2-5 molar equivalents of methanesulphonyl chloride and 2-5 molar equivalents of triethylamine are used.
 7. The process according to claim 1, wherein the reaction in step (3) is carried out at a temperature between 50° C. and 100° C. in N-methylpyrrolidone.
 8. The process according to claim 1, wherein the reaction in step (4) is carried out in a mixture of tetrahydrofuran (THF) and water with 0.01-5 molar equivalents of H₂SO₄ at a temperature between 20° C. and 100° C.
 9. The process according to claim 1, wherein in step (4) the isolation of the diol VIII is carried out by crystallization from a solvent mixture of heptane and isopropyl acetate or ethyl acetate.
 10. The process according to claim 1, wherein in step (5) 0.2-3 molar equivalents of sodium hydride and 1-2 molar equivalents of sulphonyldiimidazole are used at a temperature between −5° C. and 20° C.
 11. The process according to claim 1, wherein in step (6) 1-3 molar equivalents of tetrabutylammonium fluoride are used.
 12. The process according to claim 1, wherein in step (6) the reaction mixture obtained in the first partial step is reacted at a temperature between 20° C. and 70° C. with sulphuric acid (H₂SO₄).
 13. The process according to claim 1, wherein in partial step a) of step (7) 1-4 molar equivalents of boron trichloride are used.
 14. The process according to claim 1, wherein in partial step b) of step (7) the reaction mixture of methanol and pyridine obtained in partial step a) is reacted.
 15. The process according to claim 1, wherein the partial steps of step (7) are carried out at temperatures between 0° C. and −80° C.
 16. The process according to claim 1, wherein step (8) is carried out in the presence of 2-10 molar equivalents of pyridine using 2-5 molar equivalents of benzoyl chloride.
 17. The process according to claim 1, wherein step (9) is carried out in a solvent mixture of water and acetone using 0.5-1 molar equivalent of OXONE.
 18. The process according to claim 1, wherein in step (9) the sulphoxide XIII is isolated by crystallization from methyl tert-butyl ether.
 19. The process according to claim 1, wherein step (10) is carried out in the presence of 0.01-2 molar equivalents of potassium bisulphate using at least 5 molar equivalents of acetic anhydride.
 20. The process according to claim 1, wherein in step (10) the product I is isolated by crystallization from ethanol.
 21. A process for preparing a compound of formula I:

wherein R₁ represents —C(O)—C₁-C₆-alkyl or —C(O)-aryl; and R₂ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl, which comprises reacting a sulphoxide of formula XIII:

wherein R₁ represents benzoyl, at a temperature between 30° C. and 100° C. with at least 1 molar equivalent of an acid anhydride R₂—C(O)—O—C(O)—R₂, wherein R₂ represents C₁-C₆-alkyl, C₁-C₄-perfluoroalkyl or aryl; in the presence of 0.01-2 molar equivalents of a protic acid selected from the group of mineral acids, alkanesulphonic acids, arylsulphonic acids, perfluorosulphonic acids and perfluoroalkanecarboxylic acids, or in the presence of 0.01-2 molar equivalents of a Lewis acid, wherein R₁₃ represents C₁-C₆-alkyl or arylmethylene.
 22. The process according to claim 21, wherein the reaction is carried out in the presence of 0.01-2 molar equivalents of potassium bisulphate using at least 5 molar equivalents of acetic anhydride.
 23. The process according to claim 21, wherein the product IIβ is isolated by crystallization from ethanol.
 24. A process for preparing a compound of formula X

wherein R₆ represents C₁-C₆-alkyl, arylmethylene, which comprises the following steps: (a) a cyclic sulphate ester of formula IX

is initially reacted with 1-3 molar equivalents of an ammonium fluoride of the formula N(R₁₁)₄F, wherein R₁₁ represents C₁-C₆-alkyl, at a temperature between 0° C. and 30° C., and (b) the reaction mixture obtained in this manner is reacted with an acid selected from the group of mineral acids, alkylsulphonic acids, arylsulphonic acids, perfluorosulphonic acids and perfluoroalkanecarboxylic acids.
 25. The process according to claim 24, wherein 1-3 molar equivalents of tetrabutylammonium fluoride are used.
 26. The process according to claim 24, wherein the reaction mixture obtained in the first partial step is reacted at a temperature between 20° C. and 70° C. with sulphuric acid (H₂SO₄).
 27. An intermediate produced by the process according to claim 1, which is selected from 1-O-benzyl-3,4-O-isopropylidene-L-arabinitol; 1-O-benzyl-3,4-O-isopropylidene-2,5-di-O-methanesulphonyl-L-arabinitol; 1,4-anhydro-5-O-benzyl-2,3-O-isopropylidene-4-thio-D-ribitol; 1,4-anhydro-5-O-benzyl-2,3-O-sulphonyl-4-thio-D-ribitol; and 1,4-anhydro-5-O-benzyl-2-deoxy-2-fluoro-4-thio-D-arabinitol. 